Method For Monitoring The Operational State Of A  Surface Inspection System For Detecting Defects On The Surface Of Semiconductor Wafers

ABSTRACT

The operational state of a surface inspection system for detecting defects on the surface of semiconductor wafers is monitored by: 
     providing a reference wafer having defects of a particular number, size, and density on an examination surface; 
     conducting a reference inspection of the reference wafer and at least one control inspection of the reference wafer by the surface inspection system, the position and size of defects on the examination surface being measured; 
     identifying defects which, because of their position, are regarded as common defects of the reference inspection and of the control inspection; 
     for each common defect, determining a size difference obtained from comparing its size from the reference inspection and from the control inspection; and 
     assessing the operational state of the surface inspection system on the basis of the size differences.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to German Patent Application No. DE102014215727.7 filed Aug. 8, 2014 which is herein incorporated byreference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to a method for monitoring the operational stateof a surface inspection system for detecting defects on the surface ofsemiconductor wafers.

2. Description of the Related Art

In the course of the preparation of semiconductor wafers to formsubstrates for the production of electronic components, surfaces ofsemiconductor wafers are examined for the presence of defects. Thesurface to be examined is usually the upper side surface of thesemiconductor wafer, on which the formation of structures of electroniccomponents is intended. In order to carry out the examination, scanningsurface inspection systems may be used. These gradually illuminate thesurface of the semiconductor wafer with a light spot of laser light anddetect scattered light as a function of one or different solid angles(channels). The scattered light data obtained in this way allowinformation to be deduced about the position and size of defects whichare present on the surface examined.

So that the information about the size of defects coincides asaccurately as possible with the actual size of the defects, the surfaceinspection system is calibrated with the aid of a referencesemiconductor wafer. U.S. Pat. No. 7,027,146 B1 describes a way in whichreference semiconductor wafers can be produced. Reference semiconductorwafers are also available for purchase. A reference semiconductor waferas described in U.S. Pat. No. 7,027,146 B1 has reference defects ofdifferent size, the number and size distribution of which is known,deposited on its surface. Polystyrene latex spheres (PSL spheres) areoften used as reference defects. In the case of PSL spheres, the realdiameter of the sphere observed corresponds to the reported size of thedefect. If the reference defect does not have a spherical configuration,the size of the defect usually means its largest spatial extent.

If the surface inspection system is properly calibrated, it indicatesthe number, position and the size of the defects on the referencesemiconductor wafer with an accuracy that varies in terms of size withina specified tolerance limit (calibration tolerance). The measurementdata obtained may, for example, be processed as a histogram whichrepresents the frequency of the defects as a function of their size. Theprocessing of measurement data may be restricted to size intervals, sothat measurement data which relate to defects with a size lying outsidea size interval are not taken into account.

It is important to monitor whether a surface inspection system is in aproper operational state, and if appropriate to warn if the monitoringreveals anomalies. If anomalies occur, their cause must be investigated,and if appropriate the proper state of the surface inspection systemmust be restored. U.S. 2007/0030478 A1 describes a monitoring methodwhich provides repeated examination of a reference semiconductor waferin the course of the use of the surface inspection system. If themeasurement data of an examination of the reference semiconductor waferdo not differ substantially from those which the surface inspectionsystem delivers in the freshly calibrated state, the state of thesurface inspection system is regarded as in order. However, the scopeand sensitivity of the examinations described leaves something to bedesired. For instance, by measuring the number of defects no informationis obtained about the stability of the measurement of defect sizes as afunction of time. A possible drift of the defect size assigned to adefect by the surface inspection system is not noticed, or is onlynoticed late. In this regard, the information content remainsinsufficient, even if additional information is gathered about theposition of the maximum of the size distribution and its variation as afunction of time.

SUMMARY OF THE INVENTION

It is therefore the object of the present invention to provide acorresponding method which allows monitoring of the operational behaviorof a surface inspection system for detecting defects on surfaces ofsemiconductor wafers more comprehensively and with better sensitivity.These and other objects are achieved by a method for monitoring theoperational state of a surface inspection system for detecting defectson the surface of semiconductor wafers, comprising:

-   -   providing a reference semiconductor wafer having defects of a        particular number and size and density on an examination surface        of the reference semiconductor wafer;    -   conducting a reference inspection of the reference semiconductor        wafer and at least one control inspection of the reference        semiconductor wafer by using the surface inspection system, the        position and size of the defects on the examination surface        being measured;    -   identifying defects which, because of their position, are        regarded as common defects of the reference inspection and of        the control inspection; for each common defect, determining of a        size difference which is obtained from a comparison of its size        on the basis of the reference inspection and the control        inspection; and    -   assessing the operational state of the surface inspection system        on the basis of the size differences determined.

BRIEF DESCRIPTION OF THE DRAWINGS

Some aspects of the invention will be explained in more detail belowwith reference to the drawings.

FIG. 1 shows the size distribution of defects on a referencesemiconductor wafer.

FIG. 2 schematically shows an observation window within which a commondefect has been identified.

FIG. 3 shows a control map for monitoring of the operational state of asurface inspection system.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The method according to the invention is not restricted to observing oneor more of the following parameters and their development in the courseof operation of the surface inspection system: the number of defectsdetected, the position of the maximum of the size distribution ofdetected defects, and the width of the size distribution of detecteddefects in a specified size interval. Rather, it involves detection ofthe size of individual defects and determination of size differences ofthose defects which result from a comparison of the defect sizesaccording to the reference inspection and the control inspection. Theoperational state of the surface inspection system is assessed on thebasis of these size differences which have been determined.

The surface inspection system comprises a light source which generates alight beam, with which an examination surface is scanned, and one ormore detectors which register scattered light resulting from theinteraction between defects and the light beam. Such surface inspectionsystems are commercially available, for example from the manufacturerKLA-Tencor.

The reference inspection is preferably carried out in the course ofcalibration or immediately after calibration of the surface inspectionsystem. Before a subsequent calibration, one or more control inspectionstake place in order to obtain a picture of the operational state of thesurface inspection system and implement measures if need be, in order torestore the surface inspection system to a proper state. Between thereference inspection and a first control inspection, and optionallybetween subsequent control inspections, the surface inspectioninstrument is used for its intended purpose, namely to detect defects onsurfaces of semiconductor wafers.

In order to calibrate the surface inspection system, it is possible touse reference semiconductor wafers which have an examination surface, onwhich there are PSL spheres in a defined number and with a defined sizedistribution.

In principle, such a reference semiconductor wafer with PSL spheres mayalso be used as a reference semiconductor wafer for the referenceinspection and the control inspection, which are carried out in thecourse of the method according to the invention. The defects of thereference semiconductor wafer preferably have a continuous sizedistribution, which is difficult to achieve with PSL spheres. It istherefore preferred to use a reference semiconductor wafer with defectsthat originate from vacancy agglomerations which have been formed duringthe crystallization of a single crystal, from which the referencesemiconductor wafer is obtained. The single crystal preferably consistsof silicon. The formation of vacancy agglomerations, which may forexample be detected as COP defects, may be influenced during thecrystallization of the single crystal at an interface with a melt. Ahigh crystallization rate and a low temperature gradient at theinterface between the melt and the growing single crystal promote theformation of such defects. The examination surface of the referencesemiconductor wafer obtained from the single crystal is preferably inthe polished state. The examination surface of the referencesemiconductor wafer is that surface which is scanned in the course ofthe reference inspection and the control inspection. In contrast toreference semiconductor wafers with PSL spheres, the preferred referencesemiconductor wafer is less sensitive and can be cleaned withoutproblems. The density of the defects on the examination surface of thereference semiconductor wafer is preferably not less than 1/cm² and notmore than 15/cm².

In the course of the reference inspection and the control inspection, atleast the position and size of defects on the examination surface of thereference semiconductor wafer are measured. Common defects of thereference inspection and of the control inspection are subsequentlyidentified. These are those which can be regarded as identical on thebasis of their position. A suitable procedure for finding common defectsis described in the standard SEMI M50-0307. Accordingly, common defectsof the reference inspection and of the control inspection are suchdefects as are separated from one another in terms of their position byno more than a predefined distance (search radius) with no furtherdefect being found within this distance.

According to the invention, the size of a common defect according to thereference inspection is compared with the size of this defect accordingto the control inspection. The size difference determined by thecomparison is registered for each of the common defects identified, andthis information is used as a basis for assessing the operational stateof the surface inspection system.

This information is a reliable indicator which makes it possible topromptly recognize changes in the operational state of the surfaceinspection system, such as a drift of the defect size assigned to adefect by the surface inspection system. It is therefore preferably usedfor the purpose of statistical process control (SPC).

Such use may be carried out in a variety of ways, for example bycalculating the average size difference for one or more size intervalsof for the entire size spectrum of the common defects and registeringits development in the course of successive control inspections. Atolerance corridor, within which a registered average size difference isregarded as noncritical, is furthermore defined by a lower and an upperthreshold. If the registered average size difference departs from thetolerance corridor, this process is taken as a reason to assess theoperational state of the surface inspection system as anomalous. Whenthis event occurs, it is expedient to investigate the reasons of thedevelopment found for the average size difference without delay, and ifneed be restore a proper state of the surface inspection system.Optionally, a warning signal which signals the occurrence of this eventmay be generated. The distance between the lower and upper thresholds ofthe tolerance corridor is preferably not greater than the calibrationtolerance of the respective size interval.

Another possibility for evaluation of the information obtained about thesize difference of common defects is, for example, to plot the sizedifferences determined against the defect sizes which were measured inthe course of the reference inspection, and to define tolerance limitswithin which the status of the surface inspection system may be regardedas not anomalous.

In addition, the number of common defects identified may be registeredand the development of this parameter as a function of time may beobserved in the course of successive control inspections. Anomalies inthe development of this parameter may be taken as a reason to checkwhether the surface inspection system is still in a proper state.

As an additional measure, in the course of the reference inspection andthe control inspection, or the control inspections, the total number ofdefects may be identified and the development of the total number as afunction of time may be observed, optionally within one or more sizeintervals, in order to be able to ascertain whether the above-mentionedcalculation of the average size difference is being done on astatistically meaningful basis.

The method according to the invention is also distinguished by a certainrobustness in terms of possible contamination of the referencesemiconductor wafer in the course of the method. By virtue of thisrobustness, it is not always necessary to have to clean the contaminatedreference semiconductor wafer first, before a control inspection cantake place. Often, the result of the contamination is merely that asmaller number of common defects is identified. This result isunimportant, so long as the number of common defects identified remainssignificant for a statistical evaluation. On the other hand, a reductionin the number deemed critical may be taken as a reason to clean thereference semiconductor wafer before planned reuse.

FIG. 1 shows a typical histogram of the size distribution oflight-scattering defects (LLS) which were found on a referencesemiconductor wafer by examination with a surface inspection system in adetection channel of the surface inspection system. The frequency of thedefects is plotted against their size (DS). The reference semiconductorwafer examined is a polished semiconductor wafer of silicon. Thelight-scattering defects are defects which are attributable to thepresence of vacancy agglomerations.

FIG. 2 shows a circular observation window 1 with a radius R (searchradius), within which defects 3, 4 and 5 were found. In the examplerepresented, defect 5 has the size s₀ and the position coordinates(x₀,y₀). The index 0 is intended to indicate that the defect was foundin the course of a reference inspection. Defect 4 has the size s_(n) andthe position coordinates (x_(n),y_(n)). The index n is intended toindicate that the defect was found in the course of an n^(th) controlinspection. Defect 3 is a further defect within the radius R with aparticular position and size, which was found in the course of thereference inspection or the n^(th) control inspection. If a furtherdefect such as defect 3 is found, defects 3, 4 and 5 are not taken intoaccount because they cannot be assigned uniquely. If only defects 4 and5 are found within the radius R, they are considered as a common defect2 because of their position. According to the invention, the sizedifference is determined between the size of the common defect which isfound from the reference inspection and the size of the common defectwhich is found according to the control inspection. With reference toFIG. 2, the size difference of defects 5 and 4 is thus to be determined,if there is not a further defect such as defect 3.

FIG. 3 shows a typical control map such as may be used for monitoringthe operational state of a surface inspection system. In this controlmap, the average size difference Δ<DS> of common defects is plottedagainst the running number # of control inspections for a particularsize interval. The lower threshold LL and the upper threshold UL of atolerance corridor are also indicated.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A method for monitoring the operational state ofa surface inspection system for detecting defects on the surface ofsemiconductor wafers, comprising: providing a reference semiconductorwafer having defects of a particular number and size and density on anexamination surface of the reference semiconductor wafer; conducting areference inspection of the reference semiconductor wafer and at leastone control inspection of the reference semiconductor wafer by using thesurface inspection system, the position and size of the defects on theexamination surface being measured; identifying one or more defectswhich, because of their position, are regarded as common defects of thereference inspection and of the control inspection; for each commondefect, determining a size difference which is obtained from comparingthe size of the common defect on the basis of the reference inspectionand the control inspection; and assessing the operational state of thesurface inspection system on the basis of the size difference ordifferences determined.
 2. The method of claim 1, wherein the assessmentof the operational state of the surface inspection system is anomalousif the average of the size differences lies in a size interval outside atolerance corridor.
 3. The method of claim 2, wherein the differencebetween a lower and an upper threshold of the tolerance corridor is notgreater than a calibration tolerance in the size interval.
 4. The methodof claim 2, comprising generating a warning signal if the operationalstate of the surface inspection system is assessed as anomalous.
 5. Themethod of claim 1, wherein the defects on the examination surface of thereference semiconductor wafer have a continuous size distribution. 6.The method of claim 2, wherein the defects on the examination surface ofthe reference semiconductor wafer have a continuous size distribution.7. The method of claim 3, wherein the defects on the examination surfaceof the reference semiconductor wafer have a continuous sizedistribution.
 8. The method of claim 1, wherein the defects on theexamination surface of the reference semiconductor wafer have a densitywhich is not less than 1/cm² and not greater than 15/cm².
 9. The methodof claim 2, wherein the defects on the examination surface of thereference semiconductor wafer have a density which is not less than1/cm² and not greater than 15/cm².
 10. The method of claim 3, whereinthe defects on the examination surface of the reference semiconductorwafer have a density which is not less than 1/cm² and not greater than15/cm². 11 The method of claim 5, wherein the defects on the examinationsurface of the reference semiconductor wafer have a density which is notless than 1/cm² and not greater than 15/cm².
 12. The method of claim 1,wherein the reference semiconductor wafer is obtained from a singlecrystal and the defects on the examination surface originate fromvacancy agglomerations which have been formed during crystallization ofthe single crystal.
 13. The method of claim 2, wherein the referencesemiconductor wafer is obtained from a single crystal and the defects onthe examination surface originate from vacancy agglomerations which havebeen formed during crystallization of the single crystal.
 14. The methodof claim 3, wherein the reference semiconductor wafer is obtained from asingle crystal and the defects on the examination surface originate fromvacancy agglomerations which have been formed during crystallization ofthe single crystal.
 15. The method of claim 4, wherein the referencesemiconductor wafer is obtained from a single crystal and the defects onthe examination surface originate from vacancy agglomerations which havebeen formed during crystallization of the single crystal.
 16. The methodof claim 5, wherein the reference semiconductor wafer is obtained from asingle crystal and the defects on the examination surface originate fromvacancy agglomerations which have been formed during crystallization ofthe single crystal.
 17. The method of claim 8, wherein the referencesemiconductor wafer is obtained from a single crystal and the defects onthe examination surface originate from vacancy agglomerations which havebeen formed during crystallization of the single crystal.